It is often desirable to monitor data communications for various diagnostic information, including monitor quality of transmission, line impairments, bit error rates, modulation parameters, or the raw transferred information rate. Indeed, data modems frequently incorporate some form of modem diagnostics. In its simplest form a modem monitor is a receiver that bridges the line of a transmission and listens to the transmission.
Until recently, most data communications were half duplex or split-band. In other words, either one modem alone would transmit at any one time or two modems would occupy different frequency bands of the channel. Thus a modem monitor could alternately listen in to the transmissions of the modems engaged in data communications and monitor both directions. More recently, however, high speed full duplex, echo canceling communication techniques have become common in which a plurality of modems transmit simultaneously on the same frequency bands. Monitoring full duplex communications is more difficult than monitoring half duplex communications as the monitoring receiver must listen to and distinguish between two simultaneous transmissions in opposite directions. Prior to the present invention, no effective method or apparatus for monitoring full duplex, echo canceling was known.